Re: [NTG-context] passing dimensions to metapost

28 Aug 2008


      Thomas A. Schmitz wrote:
...
Hi all,
cleaning up the code in my module, I run into a very interesting  
problem. I want to pass a user-defined value to Metapost. Until now,  
the value "scale" had to be numeric, and I used it like this:
draw fullcircle scaled \MPvar{scale}mm
and Metapost was happy, and I was too. Now Aditya made the reasonable  
suggestion that users should be allowed to input any dimension and not  
be restricted to "mm." Easy enough. However, I also want a fallback  
just in case they input a number, in that case, I want to fall back to  
mm as a default. So I tried (after Taco's excellent explanation on the  
wiki):
\expandafter\processaction\expandafter[\CVERscale]%
      [\v!unknown=>{\doifdimensionelse%
          {\CVERscale}%
          {\setupMPvariables[CircleSomething][scale=\CVERscale]}%
          {\doifnumberelse%
      	{\CVERscale}%
      	{\newdimen\CIscale \global\CIscale=\CVERscale mm  
{\setupMPvariables[CircleSomething][scale=\the\CIscale]}}%
      	{\setupMPvariables[CircleSomething][scale=10mm\relax]%
      	   \message{Please give a proper value for the "scale" argument!}}}},
       \v!default=>{\setupMPvariables[CircleSomething][scale=10mm 
\relax]}]
So what I want: first check if \CVERscale is a dimension; if it is,  
assign it to the "scale" parameter. If it isn't, check if it is a  
number; if this is the case, append the unit "mm" to it and put it  
into a dimension register. If it isn't a number either, make the value  
sufficiently absurd for the user to realize and write a message to the  
terminal. This is all fine and dandy and works, sort of: it works  
perfectly with dimensions, but when I try with a number as input, it  
doesn't fall back to the unit "mm," but to "pt," AFAICS. That's nice  
too, but I have no clue why, and I would prefer mm. If it helps, I can  
try to give you a minimal test file tomorrow, but maybe somebody spots  
what I'm doing wrong? (I tried both with mkii and mkiv, and always get  
the same result.)
\startluacode
document.mymodule = document.mymodule or { }

function document.mymodule.checkdimen(str)
     return (str:find("%a%a$") and str) or (str .. "mm")
end
\stopluacode

\ctxlua{document.mymodule.checkdimen("#1")}

or more fancy:

function document.mymodule.checkdimen(str)
     if not str:find("%a%a$") then
         str = str .. "mm"
         ctx.writestatus("mymodule","you forgot a dimension unit, so I 
made it up for you: %s",str)
     end
     return str
end

for mkii you can use \doifdimensionelse (it reminds me to implement that 
in mkiv)

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